When a plasma process, e.g., an etching process is performed on a wafer used as a substrate, etching rates at respective portions of the wafer may be affected by temperatures of those respective portions. Thus, it has been required to maintain a temperature of the entire surface of the wafer uniformly during the etching process.
A substrate processing apparatus configured to perform the etching process on the wafer includes an evacuable chamber for accommodating therein the wafer and a mounting table (susceptor) located within the chamber for mounting thereon the wafer. Plasma is generated within the depressurized chamber and the wafer is etched by the plasma.
The susceptor has a cylindrical shape, and the wafer is mounted on the top of the susceptor. Further, a focus ring is provided on the susceptor so as to surround a periphery of the wafer mounted on the susceptor. The focus ring is made of substantially the same material as the wafer. Due to the presence of such a focus ring, the plasma can be distributed to above the focus ring as well as above the wafer, so that the etching process can be uniformly performed on the entire surface of the wafer.
When the etching process is performed on the wafer, the wafer is heated by the plasma and the temperature of the wafer may be varied resultantly. The temperature of the wafer affects a distribution of radicals in the plasma that exists above the wafer and also affects a reaction between the wafer and the radicals. Thus, if a temperature of a wafer belonging to a lot varies, it may become difficult to perform uniform etching processes on a multiple number of wafers in the same lot. Here, the susceptor includes a temperature control device, and when an etching process is performed on the wafers in the same lot, the wafers are cooled and the temperature of each wafer is controlled to a desired temperature.
When the etching process is performed on the wafer, a focus ring is also heated by the plasma and the temperature of the focus ring may also be varied. If the temperature of the focus ring varies, a temperature of a periphery of the wafer may also be varied by being affected by the focus ring. Thus, when the etching processes are performed on the wafers belonging to the same lot, the temperature of the focus ring is also controlled to a desired temperature by the temperature control device. Since, however, the focus ring is just mounted on the susceptor, adhesiveness between the focus ring and the susceptor is low and a minute gap may exist between the focus ring and the susceptor, resulting in low heat conductivity between the focus ring and the susceptor. As a result, it has been difficult to control the temperature of the focus ring to the desired temperature.
Recently, to solve this problem, the present applicant has been developing a method of controlling the temperature of the focus ring effectively by the temperature control device of the susceptor by improving the heat conductivity between the focus ring and the susceptor (see, for example, Patent Document 1). In this method, a heat transfer sheet is provided between the focus ring and the susceptor to improve the heat conductivity therebetween.
The heat transfer sheet is made of a flexible heat-resistant material such as heat-resistant silicon rubber such that the heat transfer sheet can be transformed so as to fill the minute gap between the focus ring and the susceptor. Further, for example, alumina particulates serving as fillers can be mixed in this material in order to improve the heat conductivity.
Whenever the plasma etching process is performed, the focus ring would also be etched and consumed, and if the accumulated time of the etching processes exceeds about 500 hours, the focus ring would be consumed to the extent that it is no more able to perform its function. Accordingly, the focus ring needs to be replaced whenever the accumulated time of the etching process exceeds 500 hours.
Patent Document 1: Japanese Patent Laid-open Publication No. 2002-16126
However, as compared to the focus ring of which surface is smoothly formed by machining, a surface of the susceptor is covered with thermally sprayed ceramic (e.g., alumina or yttria) and thus has high surface roughness. Further, since the heat transfer sheet is flexible, the heat transfer sheet is likely to adhere to a surface having high surface roughness. Accordingly, if the focus ring is separated from the susceptor in order to replace the focus ring, a part of the heat transfer sheet may adhere to and remain on the susceptor.
The residual heat transfer sheet adhering to the surface of the susceptor may be peeled off at a certain timing and likely to adhere to the wafer as foreign substances. Thus, the residual heat transfer sheet attached to the susceptor needs to be removed when the focus ring is replaced.
Here, since it is difficult to take the suceptor out of the chamber, the heat transfer sheet needs to be removed while the susceptor is still located within the chamber. Thus, the removal of the heat transfer sheet has been time-consuming, and complete removal thereof has been difficult.